In many advance applications there is a need to combine microelectronics with optical technologies. One cornerstone device in optical technologies is the photodetector, a device which converts a light signal into an electrical signal. The material ruling microelectronics is silicon (Si). However its optical applications are limited by its small light absorption coefficient due to its indirect band structure. Moreover the cut-off wavelength in silicon is about 1.1 μm corresponding to its band-gap of 1.12 eV at room temperature. To extend the light absorption to infrared regime, especially to 1.3 or 1.55 μm for medium or long distance communication, and/or increase light absorption, germanium (Ge) is usually introduced into silicon in the absorption region (for example, intrinsic region in a p-i-n photodiode) because it has smaller band-gap and larger absorption coefficient. It is also relatively compatible with silicon, capable of forming a good quality single crystal of Si1−xGex, where “x” is a number greater than 0 and less or equal to 1 (0≦x≦1) giving the fractional concentration of Ge. The p-i-n diode, meaning an intrinsic semiconductor layer, the “i” in the p-i-n, is disposed between two conductive semiconductors of opposite conductivity type, the “p” and the “n”. The higher the Ge concentration in the Si1−xGex alloy, the larger the light absorption coefficient at certain wavelength and the longer the cut-off wavelength will be. To an extreme, the entire intrinsic region of the p-i-n photodiode can be pure Ge. However due to the lattice mismatch between Si and Ge, to achieve defect-free quality the thickness of the Si1−xGex alloy layer grown on silicon substrate is limited by the Ge concentration, i.e. the higher the Ge content the thinner the Si1−xGex layer must be. For each Ge concentration in the SiGe there is a well defined thickness, called the critical thickness, beyond which the SiGe relaxes its strain in the form of lattice defects. Defects in the intrinsic (light absorption) region can dramatically increase the photodetector dark current and thus reduce the signal-to-noise ratio. Currently a Si based photodetector working in the 1.3 or 1.55 μm requires a Ge concentration >70% in the light absorption region, which limit the thickness of defect-free Si1−xGex<10 nm. For such a thin Si1−xGex layer, the total light absorbed, and thus the sensitivity of the photodetector is very low. For application in shorter wavelength (for example 850 nm), although higher Ge content give larger light absorption coefficient compared with Si, the limited Si1−xGex thickness will limit the benefit of adding Ge. Therefore a need exists for a structure/method for forming p-i-n diode using high quality Si1−xGex layer with large Ge concentration and large sensitivity.